A trapping prevention guard is advantageous in motor-driven seat adjusting devices in motor vehicles for safety reasons, in order to stop and possibly reverse the motorized drive when necessary, that is to say if an object or body part is trapped. Characteristic variables of the motorized drive can be evaluated in order to determine whether trapping has occurred. Such characteristic variables are, for example, the motor voltage, the motor current or the rotation speed. The motor moment can be determined from these characteristic variables, and an excess force can be determined from the motor moment in turn. The excess force is given by the difference between the total force exerted by the motor and a total adjusting force which is required, in particular, to overcome the friction and to accelerate the adjusting device. However, it is difficult to determine the adjusting force since, for example, the friction can vary during the course of the adjustment process on account of areas with severe running difficulties. In addition, aging effects or else temperature influences can have a considerable influence on the friction. Temporarily varying acceleration forces are also taken into account when determining the excess force.
According to EP 1 310 030 B1, a large number of individual forces are added up at a summation point in order to determine a resulting excess force and an excess force or a trapping force is determined by comparison with the force currently exerted by the motor.